A. Available Gas Pipelines Monitoring System
Two main methods are available today for inspection/monitoring the status of the walls in gas pipe lines, namely, optical methods and methods known as Magnetic Flux Leakage methods. Typically it is of interest to be able to determine the pipe wall thickness and other conditions of the pipe during regular inspections, preferably under normal operating conditions, and without having to take special measures, such as e.g. filling the pipe with a liquid for the purpose of providing a coupling medium for performing such measurements by ultrasonic means, since such special measures are costly and cause long disruptions in the operation of the pipeline involved. Optical methods are such as the one utilized by the “Optopig”, which is laser based and measures the distance to the wall with a resolution along and across the pipe wall of about 1 mm adapted to the inner surface, but does not measure the “remaining” thickness. The system is generally not applicable for areas covered by condensate or other liquid material. The Magnetic Flux Leakage method is a method which calculates the mass loss within a given area, but is not able to calculate absolute thicknesses, and the method is not applicable for very thick pipe walls.
It has long been stated that non-contact ultrasound (NCU) measurements of thickness and other characteristics of in a situation where a gas atmosphere exists between the measuring apparatus and the object to be measured generally is considered an impossible dream. In a pre-print of a chapter for “Encyclopedia of Smart Materials”, editor A. Biederman, John Wiley & Sons, New York (expected in 2001), by Mahesh C. Bhardwaj, published on the world wide web page http://www.ultragroup.com/Company/Publications/PDF/esmi1.pdf, that general view is emphasised. While some techniques for making NCU measurements are suggested in the aforementioned publication, they all appear to suffer from limitations to the extent that their commercial application and success in the market have not become apparent to the present applicants for patent.
Accordingly, there is a need for an apparatus and method that is simple in use, and that reliably and accurately provides NCU measurements of thickness and other characteristics of an object to be measured in a wide range of applications, and in particular for applications such as gas pipeline inspections.
B. Field of Invention
The present invention is particularly suitable for simultaneously monitoring gas pipelines for corrosion and characterize the medium outside the pipe wall. More particularly, the present invention relates to a novel apparatus and method for the in situ monitoring of such gas pipes from the inside, and at the same time characterize the medium surrounding the pipe. If the pipe is coated, the characterization could be to decide whether the coating has loosened from the pipe wall or not. The method is also applicable with some geometric limitations if there is a liquid layer covering the bottom of the pipe, the geometric limitations relates to the critical angle between the gas medium and the water surface. Above the critical angle all acoustic energy is reflected from the surface, and measurements are not possible for angles larger than this critical angle. One and the same apparatus is also applicable within the range of known offshore and onshore pipeline diameters (up to about 1.50 m).
By insonifying the pipe wall with pulsed acoustic energy comprising components with wavelengths corresponding to twice the thickness of the wall, or integral numbers of this value, these frequencies will create standing waves across the pipe wall. When the emitted pulse comes to an end, resonant energy is reradiated, and detected by a receiving transducer located at a distance from the wall.
Referring to FIG. 1 this shows an example of an acoustic signal 100 emitted from a transducer 111, reflected by a steel pipe wall 112 and received by a receiver transducer 111. Inside the pipe is a medium A, and outside the pipe is a medium B1. The acoustic signal 100 is comprised of a direct reflected part 101 and a resonant part 102. The amount of energy contained in the received signal, is influenced by the acoustic characteristics of the pipe wall as well as by the media on both sides of the wall. The closer the acoustic impedance of the medium behind the wall is to the acoustic impedance of the wall, the lower is the total reflected energy.
Referring to FIG. 2 this shows a corresponding result as in FIG. 1, only medium B2 is now different from medium B1 in FIG. 1, and as can be seen by comparing FIG. 1 and FIG. 2, the resonant part (102 and 202) of the reflected acoustic energy has changed.